This invention relates generally to a wideband RC attenuator, and more specifically to a compensation method and apparatus for such an attenuator.
An RC attenuator comprising series and shunt resistors each paralleled by a capacitor disposed between input and output terminals has widely been used to provide desired signal attenuation over wide frequencies. Such an RC attenuator is particularly useful for the input circuit of electrical test and measurement instruments such as, for example, oscilloscopes, digital voltmeters, etc. to minimize the loading effect to the signal source to be measured.
One typical example of high impedance RC attenuators is shown in FIG. 1 and comprises L-shaped arms of resistors R.sub.1, R.sub.2 and capacitors C.sub.1, C.sub.2 connected between input terminals 10a-10b and output terminals 12a-12b. The attenuation factor ATT is given by the following expression: ##EQU1##
Assuming that C.sub.1 R.sub.1 =C.sub.2 R.sub.2, then the expression (1) will be ##EQU2##
The expressions (1) and (2) suggest that the attenuation factor ATT is frequency independent if C.sub.1 R.sub.1 =C.sub.2 R.sub.2 and is determined only by resistors R.sub.1 and R.sub.2. In FIG. 2, waveforms (A) through (D) are provided to aid in understanding the attenuator in FIG. 1, wherein waveform (A) is an input squarewave signal applied to input terminals 10a-10b, and waveforms (B) through (D) are output waveforms at output terminals 12a-12b. The output will be waveform (B) when variable capacitance C.sub.1 is adjusted to meet the R.sub.1 C.sub.1 =R.sub.2 C.sub.2 condition for the flat frequency response over wide frequency range, or waveform (C) when ##EQU3## (over compensation), or waveform (D) when ##EQU4##
For accurate measurement or attenuation of the input signal over wide frequencies and various waveforms, capacitance C.sub.1 must be adjusted so that ##EQU5## One conventional technique to satisfy the R.sub.1 C.sub.1 =R.sub.2 C.sub.2 relationship is to use a variable capacitor as either C.sub.1 or C.sub.2 and manually control it so that the correct rectangular output is reproduced at output terminals 12a-12b as shown by waveform (B) in FIG. 2. If C.sub.1 is larger than the correct capacitance, sharp edges appear at transitions of the rectangular waveform as shown in waveform (C) of FIG. 2, which represents that higher frequency components of the input signal are attenuated less than DC and lower frequency components. On the other hand, waveform (D) shows the condition when C.sub.1 is smaller than the correct capacitance, thereby losing higher frequency components so as to fail to reproduce the input rectangular waveform accurately.
There are certain cases in which manual adjustment of such attenuator capacitor is impossible, difficult, or impractical. This may happen, for example, when the input voltage is very high or when there is no access to such capacitor because of a physical limitation of the equipment using the attenuator. Adjustable capacitors of high withstand voltages of 500 volts or more are difficult to manufacture, and very expensive. Additionally, adjustable capacitors may suffer variation in electrical characteristics when used in a high temperature, high moisture environment. Further, they are not suited for remote or automatic control by the aid of a microprocessor or a computer.